I figure this would make for another fun thread. Share the rarest rulers from your collection! Whether they are one of a kind or your rarest are a group of Constantine I's, they are welcome here! I'll start.
(In no particular order of rarity)
Gaius Julius Caesar, Roman Republic
AR Denarius
Obv: Diademed head of Venus, facing right
Rev: Aeneas walking left, carrying Anchises and the Palladium, CAESAR in right field
Mint: Military mint traveling with Caesar, in North Africa (struck 47-46 BC)
Ref: RSC 12, Crawford 458/1, Sydenham 1013
Majorian, Western Roman Empire
AE Nummus
Obv: DN IVL MAIORIANVS PER AVG, pearl-diademed, draped bust right
Rev: VICTORI-A AVGGG, Victory advancing right, holding palm branch and trophy
Mint: Milan (MD in ex), struck 457-461 AD
Ref: RIC X 2645
Libius Severus, Western Roman Empire
AE nummus
Obv: DN LIBIVS SEVERVS PF AVG, pearl-diademed, draped bust right
Rev: Monogram of Ricimer within wreath
Mint: Rome, struck 461-467 AD (Libius Severus reigned 461-465 AD)
Ref: RIC X 2715
Aelia Verina (wife of Leo I), Eastern Roman Empire
Obv: AEL VER-INA AVG, diademed, draped bust right
Rev: SALVS RE-PVBLICAE, Victory seated right, inscribing chi-rho on shield
Mint: Constantinople (CONE in ex), struck 457-474 AD
Ref: RIC X 656
Gelimer, Vandal Kingdom
AE Nummus
Obv: GEILAMIR, pearl-diademed, draped bust right
Rev: Monogram of Gelimer within wreath
Mint: Carthage, struck 530-533 AD
Ref: MEC 1, 28-30; BMC Vandals 4-6, Very Rare
Athalaric, Ostrogothic Kingdom
AE Decanummium
Obv: INVICT-A ROMA, Roma helmeted, facing right
Rev: DN ATHAL-ARICVS, Athalaric, in military outfit, standing, holding spear and shield, S-C across fields, X in left field
Mint: Rome, struck 526-534 AD
Ref: BMC 69, COI 85b
Theodahad, Ostrogothic Kingdom
AE Decanummium
Obv: INVICT-A ROMA, Roma helmeted, facing right
Rev: DN/THEODA/HATHVS/REX within wreath, X below
Mint: Rome, struck 534-536 AD
Ref: COI 90
Baduila, Ostrogothic Kingdom
AE Decanummium
Obv: DN BADV-ELA REX, crowned, draped, bust facing
Rev: DN B/ADV/ELA/REX in four lines within wreath
Mint: Rome, struck 550-552 AD
Ref: MIB 89a

(Tidbit for the newer members out there: I am mainly into very late Roman and post-Roman Germanic, not the most attractive coins ever but they fascinate me greatly)